PSI - Issue 33

Zhuo Xu et al. / Procedia Structural Integrity 33 (2021) 578–585

579

2

Author name / Structural Integrity Procedia 00 (2019) 000–000

Keywords: Fused deposition modelling (FDM); TPMS lattice; Thickness & scale effect; PLA; Additive Manufacturing; Compression tests

1. Introduction Additive manufacturing (AM) methods enable the fabrication of components with fully customizable shapes and mechanical characteristics, which cannot be manufactured by conventional manufacturing techniques like CNC (Brandt et al. 2013). It is a revolutionary technology for directly fabricating components from a digital CAD model utilizing a layer-by-layer material build-up process. This toolless manufacturing technology is capable of producing highly dense parts in a short amount of time with a high degree of accuracy. Moreover, it has the capability of fabricating components with complex geometries such as lattice structures, which have been widely adopted in the field of biomedical implants (Wang et al. 2018),(Kladovasilakis, Tsongas, and Tzetzis 2020),(Moiduddin et al. 2020),(Murr et al. 2012), heat dissipation applications (Wadley and Queheillalt 2007), light-weighting of aerospace components (Zhu, Li, and Childs 2018), and energy absorption for personal protective equipment (Brennan-Craddock et al. 2012). Fused deposition modeling (FDM) or fused filament fabrication (FFF) is a well-reputed one of the material extrusion AM technology. A thermoplastic filament is heated to a certain temperature and extruded through a nozzle, then the molten material from the print head is deposited on the surface of the building platform to create the 3D structures. The movement of the nozzle and building plate is controlled by the G-code files generated by the slicing software containing pre-defined printing process parameters. It is capable of fabricating components with material complexities and unprecedented geometry, such as functionally graded materials and various lattice structures. Lattice structures are porous structures composed of a periodic arrangement of three-dimensional (3D) unit cells with a pre-determined volume fraction ratio. Volume fraction can also be referred to as relative density, which is one of the critical parameters for controlling and manipulating lattice structures with varying gradients. In general, lattices can be classified into three types including strut-based lattices, solid-TPMS based lattices, and sheet-TPMS based lattices, where triply periodic minimal surface (TPMS) is a minimal surface that sometimes can be referred to as implicit-based unit cells (Al ‐ Ketan and Abu Al ‐ Rub 2020),(Benedetti et al. 2021). The minimal surface can be defined and characterized by mean curvature of zero at any point (Al ‐ Ketan and Abu Al ‐ Rub 2020). These surfaces exhibit both remarkable and unique properties. For instance, a minimal surface does not consist of any sharp edges or corners in nature. According to the literature review, the level-set approximation approach is the simplest and most extensively used technique to model 3D lattices. The level-set method (LSM) is a methodological framework that is mostly used to analyze surfaces and shapes numerically. The level-set function can be defined as ∅� � � where ∅ is an evaluated iso-surface while is an iso-value. For instance, the level-set equation for gyroid structures is listed below (Al ‐ Ketan and Abu Al ‐ Rub 2020): sin cos sin cos sin cos X Y Y Z Z X c    (1) where � � � � , � � � � , � � � � . and are all constants corresponding to the unit cell size in the , and directions, respectively. One of the widely used architectural shapes for biomedical applications is solid or sheet-TPMS based gyroid lattices discovered by Schoen in 1970. Since then, researchers have continuously demonstrated that gyroid architecture is suitable for biomorphic scaffold design in tissue engineering because of its outstanding mechanical properties. Particularly, (Kapfer et al. 2011) reported that the sheet-based gyroid lattice structures have higher stiffness than the solid-based gyroid structure at the same porosity of the same material. Similarly, (Al-Ketan, Rowshan, and Abu Al Rub 2018) investigated the mechanical properties of a wide range of structures, including strut-based, solid-based TPMS, and sheet-based TPMS porous structures. The experimental results revealed that the sheet-based TPMS structure has superior mechanical properties in terms of stress-strain responses. Although numerous researchers have already investigated the compression behaviors of various sheet-based TPMS lattice structures fabricated via different AM processes and materials. However, there is a lack of research on the size and wall thickness influence of mechanical properties for FDM fabricated specimens.